As is well known, refractory powder flame projection is now being employed as a direct repairing method for various kinds of furnaces in the iron and steel and chemical industries. It is also being used for producing refractory moldings. The flame projection moldings and the repairs made by this method have excellent properties not obtainable by conventional methods.
In the conventional refractory powder flame projecting technique, refractory particles are supplied into the stream of a projected flame which carries the particles toward a substrate into which it causes them to collide in fused state so as to form an adhering layer on the substrate. Hence easily fusable refractory particles having a relatively fine particle size, for example a size below 210 microns, are used. In this flame projection method, the fused refractory particles successively form a dense and uniform continuous solidified structure and hence the deposited layers or moldings formed possess dense structures having high strength. However, the moldings or deposits having such dense and uniform structure have certain faults:
(1) They are low in thermal shock resistance. PA1 (2) The shrinkage which results when the fused refractory particles cool and solidify causes internal stress to build up in the refractory moldings and this stress accumulates to the extent of causing breakage of the refractory molding or deposit. PA1 (3) The shrinking phenomenon causes spalling of the refractory deposits formed on the substrate by flame projection.
These faults are caused by the internal thermal stress of the refractory moldings or refractory deposits and hence the greater the surface area and/or the thickness of the moldings or deposits, the more likely the moldings or deposits are to suffer breakage. These faults being attributable to the residual stress resulting from thermal stress accumulating from the initiation of the solidification, refractory moldings and deposits formed by flame projection are, in spite of their excellent properties, limited as to the size in which they can be produced.